There are several examples of cellular and viral mRNA editing in mammalian cells. (Grosjean and Benne (1998); Smith (1997) RNA 3: 1105-23). Two examples of such editing mechanisms are the adenosine to inosine and cytidine to uridine conversions. (Grosjean and Benne (1998); Smith (1996) Trends in Genetics 12:418-24; Krough (1994) J. Mol. Biol. 235:1501-31). Editing can also occur on DNA.
A to I editing involves a family of adenosine deaminases active on RNA (ADARs). ADARs typically have two or more double stranded RNA binding motifs (DRBM) in addition to a catalytic domain whose tertiary structure positions a histidine and two cysteines for zinc ion coordination and a glutamic acid residue as a proton donor. The catalytic domain is conserved at the level of secondary and tertiary structure among ADARs, cytidine nucleoside/nucleotide deaminases and CDARs but differs markedly from that found in adenosine nucleoside/nucleotide deaminases (Higuchi (1993) Cell 75:1361-70). ADAR editing sites are found predominantly in exons and are characterized by RNA secondary structure encompassing the adenosine(s) to be edited. In human exon A to I editing, RNA secondary structure is formed between the exon and a 3′ proximal sequence with the downstream intron (Grosjean and Benne (1998); Smith (1997) RNA 3: 1105-23; Smith (1996) Trends in Genetics 12:418-24; Maas (1996) J. Biol. Chem. 271:12221-26; Reuter (1999) Nature 399:75-80; O'Connell (1997) Current Biol. 7:R437-38). Consequently, A to I editing occurs prior to pre-mRNA splicing in the nucleus. The resultant inosine base pairs with cytosine and codons that have been edited, effectively have an A to G change. ADAR mRNA substrates frequently contain multiple A to I editing sites and each site is selectively edited by an ADAR, such as ADAR1 or ADAR2. ADARs typically function autonomously in editing mRNAs. ADARs bind secondary structure at the editing site through their double stranded RNA binding motifs or DRBMs and perform hydrolytic deamination of adenosine through their catalytic domain.
APOBEC3G (alternatively referred to as CEM15) is a cytidine deaminase and APOBEC-1 homolog. APOBEC3G has been shown to possess antiviral activity. Current therapies for HIV infected patients target the production of new virus by antiviral agents that prevent replication of the viral RNA genomes into DNA prior to integration of the HIV DNA into chromosomal DNA or the disruption of the production or function of viral encoded proteins that are necessary for production of infectious viral particles. Antiviral agents that target viral replication have blunted the course of disease in patients already infected with HIV but these drugs have side effects due to toxicity and, while extending life for many patients, ultimately fail due to the high mutation frequency of HIV-1. Disruption of viral encoded protein production has not been as effective due largely to the high mutation rate of HIV and its consequence of changing the viral protein to one that retains function but no longer is a target for the therapy. A combination of therapies together with better screening of blood supplies and blood products, improved public education and safe-sex practices have curbed the spread of disease only in developed countries but, even in these countries, exhibit incomplete control over the spread of the virus. Needed in the art is a means of editing RNA or DNA involved in disease processes, like HIV, hyper-IgM syndrome, and other cytidine deaminase related diseases, thus preventing or ameliorating the symptoms, and in the case of retroviral-based diseases, eventually eradicating these diseases.